The human platelet is a major participant, normally and pathophysiologically, in coagulation and thrombosis, and can affect the contractile response of vascular tissue. Platelets also contribute to the formation of arterial plaques. Activation of platelet is dependent upon Ca-2-+ mobilization and secretion of material in platelet storage granules. Stimulated platelets display very rapid and marked alterations in phosphoinositide metabolism prior to Ca-2-+ mobilization and secretion. Increasing evidence indicate that breakdown of phosphoinositides (PtdIns, PtdIns4,5P-2) by phospholipase C (PLC) causes the mobilization of Ca-2-+ via myoinositol trisphosphate (IP-3), and activation of kinase by diacylglycerol (DG), which promote platelet morphological changes. Conversic of released arachidonic acid to prostaglandin metabolites can also participate in such activation of PLC. Utilizing techniques of capillary gas chromatography, electrophoresis, thin layer, ion exchange, and high pressure liquid chromatographies, as well as high voltage membrane permeabilization, all routinely performed in our laboratory, we will address the following questions: 1) How is PLC regulated? 2) Is the action of PLC on phosphatidylinositol (PtdIns) dependent upon hydrolysis of PtdIns4,5P-2? 3) What is the function of IP-3 in platelets? 4) Does epinephrine potentiate prostaglandin-related phosphoinositide metabolism? 5) Why is thrombin a more effective agonist for arachidonic acid release than is the prostaglandin H-2/thromboxane A-2 analogue U46619? Clarification of the factors (and their mechanisms of action) that affect phosphoinositide turnover and elucidation of the synergistic activation of platelets by epinephrine should enhance our understanding of events controlling platelet function in normal and pathological states.